The present invention relates in general to welding techniques, and in particular, to a new and useful apparatus for supplying backpurging gas to a localized area in which a welding operation is to take place.
Backpurging is generally used during welding with gas shielded welding processes to protect the backside of the weld from atmospheric gases to prevent oxidation. Backpurging becomes necessary as the surface quality requirement of the weld backside (side opposite the welding torch) increase. As the oxidation potential of a base material increases so does the likelihood that backpurging will be required. Oxidation of the backside is generally avoided by using a backpurge of an inert gas such as Argon or Helium.
Several methods of backpurging are commonly used. They are: area purge, fixed local purge and sliding local purge. An area purge consists of purging the entire backside of the area to be welded. Area purges are generally used for tubes and small vessels where the volume of backpurge gas is small. A fixed local purge is generally used on large vessels where purging the entire inside of the vessel would not be practical due to size or configuration. A fixed local purge chamber is usually made of an rigid material and sealed to the vessel with tape or an expandable bladder. A sliding local purge can be used on large vessels where a fixed local purge or area purge is not practical. The sliding local purge chamber is generally made of metal and has sliding seals which seal the chamber to the vessel.
The sliding seals used on sliding local purge chambers are generally O-rings. Referring to FIG. 1 which illustrates a typical sliding chamber seal arrangement, the typical local sliding chamber comprises a chamber wall 10 defining a concave volume 12 which receives inert backpurge gas through a shield gas inlet 14. The chamber wall includes a peripheral flange 16 which, as best shown in FIG. 1A, includes a recess 18 that confines an O-ring 20 which slides against the upper surface 22 of a member on which welding is to be performed such as a boiler vessel. The O-rings are subject to wear from sliding friction, deterioration from exposure to heat, and deterioration from ozone generated by the welding arc. This wear and deterioration causes seal leakage as welding arc time and chamber sliding time increases. This leakage in turn leads to a backpurge of lesser quality and therefore weld surface oxidation.
FIG. 1B shows an alternate prior art construction where a primary chamber wall 80 defines a concave primary gas chamber 82 having a peripheral seal arrangement generally designated 58 comprising two or more spaced seals 62, 66 and 64 separated by spacers 70 and a clamp 68 which connects the seals to the chamber wall 80. This defines a pair of void spaces 72 which receive some leakage of shield gas from the space 82 but which is subjected to wear just as the O-ring seal of FIG. 1A. Although the seal arrangement of FIG. 1B is less prone to leakage than the arrangement of FIG. 1A, the multiseal arrangement requires more room to accommodate the additional seals and raises problems of alignment for the multiple seals. Lack of alignment inherently produces a leakage problem. Since voids 72 are not pressurized, atmosphere air may leak into chamber 82.